1. Field of the Invention
The present invention relates to polyacetal compositions which contain a polyamide elastomer and are improved thereby in impact resistance and to novel stretched and oriented polyacetal-based materials having high modulus. More particularly, the invention relates to polyacetal compositions which can provide molding products having superior impact resistance, heat resistance, and stiffness and also good lubricity and surface gloss and hence are fitted as materials for use, for example, in the areas of electronics, automobiles, office automation appliances, and household appliances. Further, the invention relates to stretched and oriented polyacetal-based materials having properties different clearly from those of conventional polyacetal-based molding materials in that the former stretched materials are high in strengths such as tensile strength and tensile modulus in the direction of molecular orientation as well as high in strengths such as knot strength, hook strength, and impact resistance in the direction (lateral direction) perpendicular to the orientation direction and are also superior in surface activities such as adhesive properties; hence these stretched, oriented, polyacetal-based materials are suited especially to form, jointly with other materials, composites for use in areas wherein high strength and high modulus are necessary.
2. Description of the Related Art
Polyacetals regarded as engineering resins having a good balance of mechanical properties with electrical properties, have so far been used in various areas, but have the disadvantage of insufficient impact resistance.
Accordingly, extensive studies have been made up to now for the purpose of offsetting the deficient impact strength of polyacetals and various blends of polyacetals with polyurethane elastomers have been proposed. These proposed blends include, for example, a gloss-less polyacetal composition (Japanese Patent Application Kokai (Laid-Open) No. 59-145243) superior in impact strength made by blending 5-60% by weight of an aliphatic polyurethane elastomer having special properties and a polyacetal composition (Japanese Patent Application Kokai No. 59-155453) improved in impact resistance by dispersing 15-40% by weight of a polyurethane elastomer to fine particle sizes of 0.01 to 0.9 .mu.m in a polyacetal. However, these polyacetal compositions, though improved in impact resistance to a large extent, have lower flexural modulus, thus being impaired in stiffness, which is one of the favorable properties of polyacetal. Further, an attempt (Japanese Patent Application Kokai No. 61-19652) has been made to improve the impact resistance over a wide range of temperatures and inhibit the deterioration of flexural modulus by blending with 5-50% by weight of a polyurethane elastomer which is a copolymer of 4,4'-methylene-bis(phenylisocyanate), polyoxytetramethylene glycol, and low-molecular, straight-chain diol. In this case, however, the deterioration of flexural modulus or stiffness is also inevitable.
On the other hand, there are known polyacetal compositions resulting from blending materials besides polyurethane elastomers. An example of these compositions is a blend with an aliphatic polyether (Japanese Patent Publication No. 50-33095), another example of the compositions is a blend with a copolyester (Japanese Patent Publication No. 60-19331), and other examples of the compositions are blends with polyamide elastomers (Japanese Patent Application Kokai Nos. 59-191752 and 61-183345). However, none of these compositions are improved much satisfactorily in both impact resistance and flexural modulus. The above Patent Application Kokai No. 59-191752 suggests mixing of a commercial nylon-12 elastomer with a polyacetal to impart antistatic properties thereto. However, the present inventor's experiment tracing this composition reveals that it is scarcely improved in impact resistance. In addition, this blending enhances but insufficiently the antistatic properties.
As stated above, no polyacetal composition has yet been found out that is superior in both impact resistance and stiffness. Hence the development of such a polyacetal composition has been strongly desired.
Recently, polyacetals have also been studied for the purpose of applying them especially as raw materials of composites for use in areas wherein materials of high strength and high modulus are necessary.
The method of heating and stretching a polyacetal in the air (Japanese Patent Application Kokai No. 50-77479) has so far been known as an example of means for producing polyacetal articles of high modulus. According to this method, a tensile modulus as high as 49 GPa can be achieved, but voids form in the polyacetal and its fibrillation takes place during the stretch. Hence this method has a drawback in that as the modulus is increased, the apparent density and the strength decrease inevitably. For example, the present inventors experiment tracing this method has confirmed that as the tensile modulus of polyacetal is increased up to 42GPa by stretching it, the apparent density thereof decreases from 1.42 g/cm.sup.3 before stretch to 1.15g/cm.sup.3 after stretch and the tensile strength decreases to 1.1 GPa and that the stretch results in not only such decrease in stretch-directional mechanical strength but also decrease in transverse mechanical strength, that is to say, the stretch tends to give rise to buckling, napping, splitting, and tearing.
For the purpose of producing the stretched and oriented polyacetal materials superior in mechanical strength not only in the stretch direction but also in the transverse direction, the present inventors, previously stretched a polyacetal under heated and pressurized conditions by using a special means, whereby the transverse mechanical strength of the stretched polyacetal was found to improve outstandingly (Japanese Patent Application Kokai Nos. 60-183121 and 60-183122). However, common polyacetals before stretch are chemically inactive by nature and after stretch are more inactive and therefore the material stretched as stated above had the drawback of being difficult to form composites.
Meanwhile, the addition of other compounds to polyacetals has been tried for the purpose of modifying the polyacetals. As an example, a polyacetal composition containing a polyurethane elastomer is expectable to be improved in not only impact resistance but also surface activity since the polyurethane elastomer is polar. Accordingly, the present inventors tried to stretch such the polyacetal composition, but tensile modulus and tensile strength of the stretched polyacetal were found to be about 10 GPa and about 0.7 GPa, respectively, thus being unsatisfactory for practical use, though an increase in the surface activity was observed.
Another known polyacetal-other compound composition is a blend of polyacetal with a polyamide elastomer consisting of nylon-12 segments and polytetramethylene oxide segments (Japanese Patent Application Kokai Nos. 59-191752, cited above, and 61-183345), but the stretch of this composition is not tried. As can be seen also from the above stated stretch experiment, however, it is readily estimated that although elastomer-containing polyacetals, either unstretched or stretched, will be provided with so-called rubber elasicity (viscosity), in other words, impact resistance and flexibility that are properties of elastomers, so-called crystallinity (rigidity), in other words, the stiffness and strength of such polyacetal compositions, on the contrary, will be impaired.
While a great number of polyacetal compositions improved in rubber elasticity or hydrophilicity by blending various additives are known other than the above stated compositions, said additives are, so to say, impurities and hence may cause breaks of the compositions under stretch, so that no high modulus or high strength can be expected.
As stated above, it has been tried recently to use stretched polyacetals in areas wherein materials of high modulus or high strength are necessary. Depending upon their applications, the use of stretched polyacetals in composites are possible. Accordingly, stretched polyacetal materials having high modulus, high strength and surface activity are looked for, but none of such materials have yet been discovered.